K-Lite Codec Pack 64-bit is a bundle of 64-bit DirectShow filters that can be used together with 64-bit players, such as Vista Media Center. This pack can co-exist without problems with the regular (32-bit) K-Lite Codec Pack.

The 64-bit versions of Windows can run 32-bit software perfectly fine. So it is recommended to use the regular K-Lite Codec Pack when possible. That one contains more, better and faster decoders. You only need 64-bit filters for use with 64-bit applications.